AMMA Water Cycle Work Group (WG2)

Case study of moving precipitating systems: Associated water atmospheric and hydrological budgets

Open to the whole AMMA Community

Coordinators : C. Peugeot and J.-L. Redelsperger

Presentation

Motivations -Objectives - Case Description - Methods

The full description of the exercise is available in the "Documents" section (click here for whole document in pdf)

Motivations

A privileged scale where most of disciplines and models can be integrated is the Mesoscale (10³ - 10⁵ km²) as

Ø complying with both hydrologic and atmospheric model capabilities

Ø being the scale of MCS & many surface-atmosphere interactions governing the transports of water vapour from the lower atmosphere to the deep atmosphere and controlling the timing of convection and precipitation

Ø benefiting the most directly from enhanced observations (high frequency soundings, surface flux network, radar observations, ...).

In the framework of AMMA Water Cycle Work Group (WG2) setup by ISSC (International Scientific Steering Committee), a first case study is proposed to compare between them and with observations atmospheric and hydrologic simulations at the meso-scale. It constitutes a first step in science integration in preparing further studies based on SOP observations (tools and methods testing). The main motivation comes from the need to validate model outputs and to ensure that outputs of atmospheric models comply with forcing of hydrologic models, and vice-versa.

Objectives

The main objective is to run on a same case study, meso-scale atmospheric models (MSAM) with explicitly represented convection, and meso-scale hydrologic models (MSHM). The comparison of the two approaches between them and with observations is a necessary challenge to tackle the questions related to meso-scale water cycle as raised in the AMMA International Science Plan (ISP, 2005).

This case is also an excellent way to evaluate representation of water cycle in global and regional models. In particular this case is part of the “dry run” of AOC (AMMA Operation Center) forecast. Many Numerical Weather Prediction (NWP) models participate to this exercise showing difficulties to represent the observed precipitating convective system. If results on the present case study are enough good quality, the case could be proposed as a case study as well for Atmospheric Single Column Models (ASCM) coupled with a surface representation.

Brief description of the case

A MCS “A” formed over central Africa (Southern Sudan/Chad) the 27^th Aug 2005 around noon. The MCS moved first northwards over the Chad and then westwards (Nigeria) centred along the 10 North latitude during the night. The 28^th Aug in the morning, a new MCS “B” was initiated over the north part of Jos Plateau (North Nigeria) near MCS “A” (northwest from its leading edge). The case-study will focus on this second MCS “B” rapidly growing, propagating westward, and passing over Benin between 16:00 and 21:00 UTC, and then over Ghana where it dissipates. A new convective element (named “C”) was initiated on the north part of MCS “B” at 16:00UTC along Nigeria and southern Niger limit. It moved across Niger during the night and developed in moving westward over Burkina Faso.

Methods

1. MSAM runs : The objective is to provide the more realistic MSAM precipitation field to force MSHM.

Atmospheric fields to initialize MSAM derived from NWP analysis, as well as lateral boundary conditions for limited area models : ECMWF analysis was chosen for reference simulations

At the same time, comparisons of analyses from different NWP centers, between them and with available observations in order to assess their quality. As NWP analyses may differ, tests will be necessary to assess the impact of using different analyses on MSAM results.

2. MSHM runs: The objective is to simulate the hydrologic impact of the precipitating system, as observed on the 3 mesoscale sites (runoff, groundwater refilling, soil moistening, evaporation and transpiration)

Forcing with observed rainfields and Potential Evapotranspiration (ETo)

MSHM initialization with the moisture state of the system (soil and groundwater) ; use of the driest moisture status at the beginning of each season ; simulation of the whole season, and extraction of the time-period of the case-study.

No data assimilation with the participating models, nor explicit vegetation functioning (forcing with rainfall and ETo)

3. MSAM and MSHM evaluations in a coupling perspective: At this stage, fine scale evaluation of atmospheric and hydrologic simulations over the common domain (meso-scale sites) will be undertaken.

MSAM: analysis of timing, propagation, intensity and smaller scale structures (e.g. cell statistical properties) of the simulated precipitating system. Regarding specific hydrological needs, this translates into rainfall statistics (intensity, space-time distribution) and impact on the low levels of atmosphere (wind, temperature, moisture, cloud cover) including changes in surface evaporation and the surface energy budget.

MSHM: comparison of the simulated components of the continental water cycle to observed dataset, focusing on the following key variables : river runoff at basin and/or at sub-catchments outlet ; groundwater level ; evapo-transpiration (point observations from flux stations, other products) ; soils moisture (satellite or ground observations). As none of the participating hydrological model explicitly uses a SVAT module, comparisons with energy budget data (temperature, radiation ) will not be considered

MSAM/MSHM intercomparison: This intercomparison will focus on variables simulated by both types of model, viewed as ;interface variables between the surface and the atmosphere : evapo-transpiration (MSHM) vs latent heat flux (MSAM) ; Soil moisture. These comparisons would require to use up/downscaling techniques.

4. MSHM runs forced by MSAM rainfields: If previous steps are considered enough successful, MSHM will be ran in using rainfall issued from MSAM experiments. These experiments will require rainfall downscaling techniques.

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updated : April 19th 2006